Metal cyclopentadienide generally indicates potassium cyclopentadienide (KC5H5) or sodium cyclopentadienide (NaC5H5).
The potassium cyclopentadienide was first synthesized by the reaction of cyclopentadiene (Cp) and potassium (K) 100 years ago. In general, the potassium cyclopentadienide can be synthesized by reacting cyclopentadiene with potassium metal in the presence of tetrahydrofuran (THF), benzene or liquid ammonia.
The synthesis of the sodium cyclopentadienide was first reported by K. Ziegler and E. O. Fisher. And the synthesis of the sodium cyclopentadienide these days is largely induced by the reaction of cyclopentadiene and NaOH, tert-NaOtBu or NaCl.
The conventional synthesis methods all use cyclopentadiene as a reactant. The cyclopentadiene is in equilibrium with dicyclopentadiene and most of the time it remains as dicyclopentadiene at room temperature.
So, the cyclopentadiene is generally obtained from dicyclopentadiene by retro Diels-Alder reaction at 170° C. However, the synthesis method of metal cyclopentadienide by the retro Diels-Alder reaction using the dicyclopentadiene as a reactant has such problems that it is difficult to control the Diels-Alder reaction with cyclopentadiene, suggesting that dicyclopentadiene might still remain, and a process of producing cyclopentadiene has to be included, making the procedure complicated.
There has been an attempt to synthesize metal cyclopentadiene not by the indirect reaction with cyclopentadiene but by direct reaction with dicyclopentadiene.
Particularly, it has been tried to synthesize metal cyclopentadienide using dicyclopentadiene as a reactant in a high temperature/high pressure reactor or by the reaction with sodium metal in the presence of iron at room temperature or at 170° C.
However, the synthesis of metal cyclopentadienide using the high temperature/high pressure reactor or via reaction using iron as a catalyst does not favor mass-production.
As an alternative, a method to react dicyclopentadiene (as a reactant and reaction solvent) directly with sodium or potassium was reported in Organometallics 2003, 22, 877-878. However, this method also has a problem of over-dose of dicyclopentadiene which is necessarily used for mass-production but has to be eliminated.